1. Field of the Invention
The present invention relates to a reading method and a reading apparatus for optically reading stacked bar code information.
2. Description of Related Art
Conventionally up to now, bar codes are used prevailingly as a means in which a medium such as paper, a plastic material, and the like is provided with specific information and the information is automatically recognized. For example, one-dimensional bar codes are used for general consumer products. In the coming years, as a greater amount of information is requested for such a bar code, two-dimensional bar codes have appeared, being accompanied with their information capacity increased to be significantly greater than the one-dimensional bar codes have. Then, reading apparatuses available for those two-dimensional bar codes are also becoming widespread by degrees.
Among those two-dimensional bar codes, there is a type of bar code in which the information capacity is increased by means of stacking one-dimensional bar codes, and eventually this type of bar code is called the ‘stacked bar code.’ A typical kind of stacked bar code is a symbology system called PDF417, which is internationally standardized as ‘ISO/IEC15438.’
Reading apparatuses are described, for example, in Japanese Unexamined Patent Application Publication No. H09-022438 (hereinafter “Patent Document 1”) and Japanese Unexamined Patent Application Publication No. H08-241369 (hereinafter “Patent Document 2”). In the case of an apparatus for reading stacked bar code information disclosed in Patent Document 1, a two-dimensional image pickup device captures an image of a bar code at first. Then, having read code words as many as possible, the reading apparatus makes corrections so as to enable shortening a processing time. In the case of an apparatus for reading stacked bar code information disclosed in Patent Document 2, a two-dimensional image pickup device captures an image of a bar code for understanding the degradation state of the bar code.
In the case of the apparatus for reading stacked bar code information disclosed in Patent Document 1, the number of all code words constituting the bar code to be read is compared with the number of code words that have been read successfully. If the number of code words that have been read successfully is greater than the number based on a predetermined percentage, corrections are made. Meanwhile, if the number of code words that have been read successfully is less than the number based on the above percentage, the image pickup device captures another new image, which is then read. This process is repeated until the number of code words that have been read successfully exceeds the number based on the above percentage. Therefore, depending on the state of the medium, the condition for moving to the step of making corrections may not be satisfied, or a considerable number of retry operations as described above may be needed before satisfying the condition. Eventually, such a configuration that capturing an image takes much time unfortunately leads to a long processing time.
In the case of the apparatus for reading stacked bar code information disclosed in Patent Document 2, it is judged whether or not error corrections for the bar code have been made rightly. If making the error corrections finishes adequately, operation moves to a step of converting data. However, if the error corrections cannot be made, another image of the bar code is captured again. Therefore, in the case where the condition for making the error corrections is not likely satisfied due to a stain or distortion of the medium, the number of retry operations for capturing another image unfortunately increases to lead to a long processing time.